Musnad Ahmad 1153:476

Zakariya ibn 'Adi narrated to us, he said: Ibn al-Mubarak informed us, from Yunus, from al-Zuhri, he said: 'Urwah narrated to me, from 'Aishah, she said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever catches a prostration of the 'Asr prayer before the sun sets, and of the Fajr prayer before the sun rises, then he has caught it."